<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
		<style>
			#box{
				width: 300px;
				height: 260px;
				background: blue;
				/*注意!!!!margin:0 auto;只能让块级元素在父级范围内水平居中*/
				margin: 0px auto;
			}
			
		</style>
	</head>
	<body>
		<div id="box"></div>
		
	</body>
</html>
